1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish, and more particularly to a polishing apparatus having a pusher for transferring a workpiece between a top ring of a polishing apparatus and a robot associated with the polishing apparatus.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the top ring and the turntable are rotated.
It has been customary to install a robot to transfer a semiconductor wafer therefrom to the top ring before it is polished, and to transfer the semiconductor wafer from the top ring thereto after it is polished. That is, the semiconductor wafer is transferred directly between the top ring and the hand of the robot which is associated with the polishing apparatus.
However, to transfer a semiconductor wafer directly between the top ring and the hand of the robot causes a conveyance error because the top ring and the robot usually have irregularities in conveying accuracy.
In order to improve the conveying accuracy of the top ring and the robot, it is preferable to install a pusher at a transfer position for the semiconductor wafer. In this case, the pusher has such a function as to place thereon a semiconductor wafer to be polished, which has been conveyed by the hand of the robot, and then to lift and transfer the semiconductor wafer onto the top ring which has been moved over the pusher. Further, the pusher has another function as to receive the semiconductor wafer which has been polished from the top ring, and then to transfer the semiconductor wafer onto the hand of the robot. Therefore, the pusher can smoothly transfer the semiconductor wafer between the top ring and the hand of the robot without a conveyance error.
The pusher, however, has to be positionally adjusted highly accurately with respect to both the top ring and the hand of the robot. Troublesome and time-consuming operations are required to set an accurate transfer position where the pusher is to be positioned.